22,984 reputation
152123
bio website lightandmatter.com
location Fullerton, California
age
visits member for 3 years, 2 months
seen 9 hours ago

I teach physics at Fullerton College, a community college in Southern California. I have an undergrad degree in math and physics from Berkeley and a PhD in physics from Yale. Back when I was doing research, my field was experimental low-energy nuclear physics.


9h
comment Would an ideal gas be colder at higher altitude due to gravity?
The answer is no. This was a controversy in the 19th century. The putative effect can be referred to as the Loschmidt gravito-thermal effect. Boltzmann, Maxwell, and Guthrie debated the question.
14h
comment Why most problems encountered in quantum mechanics cannot be solved exactly?
If you expect closed-form solutions to be common, then you're going to be disappointed in all areas of physics. For example, even the three-body problem in Newtonian gravity has no closed-form solution.
18h
comment Can we prove Conservation of Angular Momentum without assuming internal forces are central?
Conservation of angular momentum can't be proved from Newtonian mechanics, because conservation of angular momentum is true in general, but Newtonian mechanics is not true in general. Conservation laws are fundamental. Newton's laws are not.
22h
comment How come we talk about gravitational potential energy and not gravitational potential?
The metric is the central object of study in general relativity, and the metric is essentially the gravitational potential.
2d
comment Can gravitational waves be red-shifted?
@arc_lupus: Cite: "Yes, gravitational waves will undergo the same red-shift as any wave that propagates at c." This is simple kinematics. It has nothing to do with the specific characteristics of the wave. For an example of how this is derived, see section 3.2 of my SR book: lightandmatter.com/sr
2d
comment Can gravitational waves be red-shifted?
@arc_lupus: Isn't this true for all waves without respect for their speed? No, it can't be. If a wave travels at some velocity v that is less than c, then v depends on the frame of reference, and the Doppler shift can't just be a function of the emitter's state of motion relative to the receiver. For example, the Doppler shift of a sound wave depends on the emitter and receiver's states of motion relative to the vibrating medium.
2d
comment Is it correct to say “like poles attract, unlike poles repel” while two magnets are placed such that one is inside another?
Magnets don't actually have poles. Magnetic monopoles don't exist: en.wikipedia.org/wiki/Magnetic_monopole
2d
comment How do we know that there is more than one photon in the universe?
A photon is massless and travels with lightspeed, which means for the photon itself, that time stands still and space shrinks to zero. Not true. See physics.stackexchange.com/questions/29082/…
2d
comment How do we know that there is more than one photon in the universe?
Photons simply make a full rotation from the time-like to the space-like coordinate axis. No, this is wrong.
2d
comment Impact force in a fall
@Qmechanic: Thanks for pointing me to that info. After reading it, I still don't think this should have the homework tag.
Sep
29
comment Why $r^2$ and not other power of $r$ in mutual force of gravitation?
This is fine as a heuristic argument, but as a matter of taste, one may or may not like it. Gravity isn't a physical substance that spreads out over an area like peanut butter.
Sep
29
comment Why $r^2$ and not other power of $r$ in mutual force of gravitation?
You've asked an intellectually sophisticated question, and you deserve an intellectually sophisticated answer. The trouble is that there is no uniquely defined answer to this type of "why" question. If there's an answer that you will find satisfactory, it will be an answer that appeals to something you consider more fundamental. But we don't know what you'd say was more fundamental than this law. That's a matter of taste. Empirical observations? General relativity? Heuristic arguments about the commonness of inverse-square laws, because we live in three dimensions and area goes like $r^2$?
Sep
29
comment Tensor equations in General Relativity
@lurscher: Right, I think we're in agreement. I was trying to be nontechnical in my answer, but basically everywhere that I say "coordinates," technically it should be "atlas."
Sep
29
comment Impact force in a fall
@kenik: Since you're a climber, you know about static vs dynamic ropes. The whole reason for using dynamic ropes is to reduce the force of impact. This is why the equation in the answer depends on quantities like the Young's modulus. The Young's modulus for a dynamic rope is small, so the impact force is small.
Sep
29
comment Impact force in a fall
This doesn't actually look like homework, so I'm removing the homework tag.
Sep
29
comment What was Feynman's “much better way of presenting the electrodynamics” — which did **not** appear in the Feynman lectures?
The question is about electrodynamics, not quantum mechanics.
Sep
29
comment Spacetime Torsion, the Spin tensor, and intrinsic spin in Einstein-Cartan theory
If torsion is going to be interesting, then you have to have something that acts as a source of torsion. The idea of letting the spin-1/2 of fermions act as its source is only an assumption. Good question, though, as to why orbital angular momentum can't be a source. I'd like to see an explanation of that. It might simply be that orbital angular momentum is easily ruled out empirically.
Sep
29
comment Tensor equations in General Relativity
Tensorial relationships are form-invariant under a change of coordinates. In SR, a choice of coordinates can be identified with a choice of observer, but that's not the case in SR.
Sep
28
comment When does a singularity start to exist during a black hole formation?
The "when" part of the question has no definite answer. Simultaneity isn't well defined in general relativity, and it isn't even well defined for a particular observer in a particular state of motion, as it would be in special relativity.
Sep
28
comment When does a singularity start to exist during a black hole formation?
It is the existence of the even horizon that defines the black hole "singularity". No, the event horizon and the singularity are two different things.